Plain or standard glass is a 2mm clear glass and our most common used. It will protect your item from dust and bugs and if your frame is hung in the right spot it is fine. You may get a reflection from the glass if the light is shinning on the frame and a print can fade over time pending on ink and paper quality when using this glass but this is the most affordable option.
UltraVue glass is an amazing product. It may cost a bit more but for its price the results certainly justify it. This glass is almost invisible, and works even better when the artwork is set back a bit with a spacer or fillet. UltraVue is also has 70% UV protection for you artwork to prevent fading or sun damage to you paper.